Responses of montane forest to climate variability in the Central Himalayas of Nepal
Recent studies have shown that temperature increase rate in Himalaya region is higher than the global average and precipitation patterns have been more erratic. Climate changes have multiple ecological impacts to Himalaya vegetation, where multiple factors like growing season length, moisture, and ambient temperature limit the growth conditions. Vegetation pattern in Himalaya is very diverse which results into differential relation with the climatic factors. This research documents the impact of climate variability and change on five different forest stands with different dominant tree species in Manaslu Conservation Area, Central Nepal. We classified landcover using Landsat data and selected five forest stands using landcover data, existing forest classification and field expedition. MODIS Normalized Difference Vegetation Index (NDVI) of Betula-Abies, Larix sp., Pinus wallichiana, Quercus semecarpifolia, and Picea-Tsuga forest stands were extracted to analyze temporal trend of productivity and to compare relative productivity among them. Mean monthly surface air temperature and precipitation data of nearest weather station were used to analyze response of different forest stands to current climate variabilities, as well as to offer some insights into the trajectory of these changes over time.
Relations differ according to forest type and shows that both precipitation and temperature affect monthly NDVI values in most of the forest types, although we record more significant correlations with temperature data. Betula-Abies, Larix, and Pinus wallichiana forests had a greater correlation with the average temperature of the month preceding the NDVI measurement, while Picea-Tsuga and Quercus semecarpifolia forests showed greater correlations with the temperature of same month. These results clearly depict vegetation's strong response to temperature and raise new study questions regarding the types of temperature change that will have the greatest impact as global warming increases. On the other hand at the maximum increased temperature there was still an ongoing increase in vegetative vigor. This shows that temperature still is the major limiting factor for plant growth at higher elevation sites. This part of the Himalayas has abundant moisture and some forest types are already saturated in terms of growth in relation to precipitation. Clear increase in productivities is documented on the upper tree line ecotones, and these systems are likely to continue to have increasing growth rates despite use of forest resources by local people. Perhaps the local climate changes are effectively compensating for increased human use.